Our electrophysiological studies have provided evidence for the concept of a special sub-organization among members of a muscle's group Ia spindle afferents and its homonymous alpha motoneurons. We have also been able to conduct a limited histological study which has provided some anatomical information corroborating the conclusions of the electrophysiological studies. At present, it appears that intramuscular nerve branching may be a basis for segregating subsets of a muscle's efferent nerve fibers into isolated groups whose members show stronger interactions if they reside in the same nerve branch. However, a more detailed investigation into the anatomical organization of the relevant subsets of motoneurons, motor units, and spindle afferents will have to be carried out before our present hypothesis can be established as an acceptable principle of segmental sensory-motor organization. The present proposal, therefore, is a plan to integrate into our present research effort the full histological capability for conducting those critical anatomical experiments necessary to correlate with the present electrophysiological studies. The peripheral organization of spindle afferents and motor units in an isolated intramuscular nerve branch will be studied by transecting all other nerve branches to the muscle and electrically stimulating the remaining motor axons until all glycogen is depleted from the active muscle fibers. Subsequent analysis of serial sections stained alternately with PAS, Van Gieson, and H & E techniques will allow reconstruction of the innervated region of muscle and determination of the number and location of spindles within that area. Anatomical localization of motoneurons within the spinal cord and spindle afferents within the dorsal root ganglia with respect to their intramuscular nerve branch of origin will be studied by retrograde chromatolysis techniques. Finally, the location of physiologically studied motoneurons (intracellularly marked with procion yellow) whose isolated activation mechanically elicited specific spindle afferent activity will be determined from serial section reconstruction of spinal cord sections. Concurrent analysis of anatomical questions pertinent to electrophysiological investigations are needed to provide a rational foundation for the establishment of principles underlying sensory-motor organization.